Max Planck Institute for Astronomy - Annual Report 2005

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56 III. Scientific Work The Field of the Large Magellanic Cloud with HST Observations We were recently able to present the first evidence for flattening of the IMF toward the low-mass regime in the field of the LMC (Gouliermis, Brandner & Henning 2006, ApJ, 641, 838). In this study, a large amount of archived HST/WFPC2 photometric data of an area located to the west of the Bar of the LMC, was used. This area accounts for the general background stellar field of the inner disk of the LMC and the data cover six overlapping WFPC2 fields, reaching magnitudes as faint as V � 25 mag, and providing a large sample of more than 80 000 stars. This data enabled us to determine the PDMF of the main-sequence stars in the field of LMC in detail down to about 0.7 M 0 . This mass function is identical to the Initial Mass Function (IMF) for stars with masses lower than about 1 M 0 , since no evolutionary effects had time to take place and alter the initial masses of these stars, or remove them from the main sequence. The highest mass found in the data is about 2 solar masses, and the PDMF of main sequence stars up to this limit in the LMC field is found not to have a uniform slope throughout the observed mass range, meaning that log [� (/log M/kpc 2 )] 7 6 5 4 –0.2 –0.1 Reconstructed IMF of all stars IMF of red giants PDMF of main-sequence stars 0 0.1 log [M/M � ] 0.2 0.3 0.4 Fig. III.1.7: The Mass Function of the stars in the general field of the Large Magellanic Cloud to the west of its bar, observed with HST/WFPC2. The Present-Day Mass Function (PDMF) of the main-sequence stars was plotted with the green dotted line. Several assumptions concerning the star formation history of the LMC were taken into account for the reconstruction of the Initial Mass Function of these stars. The IMF of the Red Giant Branch stars (RGB) was constructed after taking the evolutionary effects of these stars into account and was plotted with the red dashed line. The small number of RGB stars in the area and their narrow mass range introduce only small changes to three specific mass intervals (red arrows), showing a trend of the IMF to become slightly more shallow toward the limit of about 1.6 M 0 . The IMF of all stars (both red giants and main sequence stars) was plotted with the blue solid line. It was reconstructed by the combination of the IMF of main sequence and RGB stars. All mass functions shown were shifted to avoid overlapping. the PDMF slope of the LMC field does not follow a single power law. This complies with what we found for the general background population in the case of the LH 55 field and LH 52 Area. We established that a multi-power law can represent the field PDMF, the slope of which changes at about 1 M 0 to become shallower for stars with smaller masses down to the lowest observed mass of about 0.7 M 0 , and show clear indications of flattening for even smaller masses. We statistically verified that the IMF has a slope Γ of roughly – 2 for stars with masses lower than 1 M 0 , with a Salpeter-like slope Γ � – 1.4 for stars in the mass range between 0.7 and 0.9 M 0 , while for more massive stars the main sequence PDMF becomes much steeper with Γ � – 5. The main-sequence luminosity function (LF) of the observed field closely matches the previously found Galactic LF. Taking several assumptions concerning evolutionary effects into account, which should have changed the stellar content of the observed field through time, we qualitatively reconstructed its IMF for the whole observed mass range (0.7 – 2.3 M 0 ). We found that the number of observed evolved stars is not large enough to have significantly affected the form of the IMF, which is almost identical to the observed PDMF (Fig. III.1.7). Deeper observations would certainly provide a better statistical stellar sample with lower masses to verify this result. The Pre-Main Sequence Population of Associations in the Magellanic Clouds Low-mass stars (with masses lower than 1 M 0 ) are very important for addressing some of the most fundamental problems in star formation, as they provide a snapshot of the fossil star formation record of giant molecular cloud complexes. Large scale surveys have identified hundreds of low-mass members of nearby OB associations in the Milky Way, and revealed that lowmass stars exist wherever high-mass stars have recently formed (Briceño et al. 2006, in Protostars & Planets V). The low-mass stellar populations of Galactic OB associations are considered to be a key to investigating fundamental issues in the formation and early evolution of stars. The shape of the low-mass IMF is still a debated issue, and whether OB associations have low-mass populations according to the field IMF of the Galaxy, or if their IMF is truncated. Infrared studies have provided strong/convincing evidence of primordial accretion disks around low-mass stars, which dissipate on various timescales of 1 to 20 or even 30 million years. These are infant stars which are not yet on the main sequence and therefore named pre-main- sequence (PMS) stars. Do such stars exist in associations of the MCs? An answer to this question is only possible with observations from instruments equipped with high resolving power. Our study using HST observations of the association LH 52 revealed a low-mass stellar population which is
considered to be the PMS population of the association (Gouliermis, Brandner and Henning 2006, ApJL, 636, L133). This is the first time such stars were found in the vicinity of an extra-galactic association. The candidate PMS stars form a secondary red sequence, running almost parallel to the lower main sequence from the turnoff down to the detection limit, as it is seen in the CMD of LH 52 from the WFPC2 observations (Fig. III.1.8). Almost 500 candidate PMS stars were selected. PMS evolutionary models of ages between 1.5 and 15 million years (Siess et al. 2000, A&A, 358, 593) trace the location of these stars in the CMD very well, providing the first evidence that these stars are of a PMS nature. Comparisons of the CMD of stars with high statistical significance in the area of LH 52 to the ones in different areas of the field of the LMC showed that these stars are merely part of the association and they are not a common general feature of the LMC. This indicates that the suggestion/theory that the secondary red sequence represents PMS stars in the association and not low-mass stars on its main sequence. Furthermore, the location of the candidate PMS stars in the CMD almost perfectly matches the loci of low-mass PMS stars recently discovered in the Galactic association Orion OB1 (Sherry et al. 2004, AJ, 128, 2316; Briceño et al. 2005, AJ, 129, 907). The latter stars are identified as T Tauri stars with masses Fig. III.1.8: top right and bottom: Color-Magnitude Diagram of the low-main sequence stars in the observed WFPC2 field of the association LH 52. In the upper plot, Pre-Main Sequence isochrone models for ages 1.5, 2.5, 5, 10, and 15 million years and for the metal abundance of the LMC are plotted with blue dashed-dotted lines. They track the discovered secondary faint red sequence well, which does not seem to coincide with either a binary sequence or main-sequence broadening due to photometric errors. The candidate low-mass PMS stars are represented with thick red dots. The models of Zero Age Main Sequence V [mag] 22 23 24 25 26 27 1.5 M � 1.4 M � 1.3 M � 1.2 M � 1.1 M � 0.9 M � 0.8 M � V [mag] 0.7 M � 1 1.5 V – I [mag] 20 21 22 23 24 25 26 III.1 Star Formation in the Magellanic Clouds 57 0.5 1 1.5 V – I [mag] and birth line are plotted with thick black lines. The horizontal blue lines on the right indicate typical mean photometric errors in color. In the lower plot, PMS evolutionary tracks for masses down to 0.3 solar masses are overplotted on the diagram of the candidate PMS stars. These tracks are indicative of the corresponding mass of each star and they are used for the construction of the IMF of these stars. The corresponding mass is indicated next to each track. On the right, the detection efficiency (in %) in each brightness range (completeness) is also provided. 0.6 M � 0.5 M � 0.4 M � 2 2.5 0.3 M � 100 % 99 % 98 % 97 % 86 % 56 % 14 % 6 % 2
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Max Planck Institute for Astronomy
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Max Planck Institute for Astronomy
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Contents Preface ..................
Page 8 and 9: 6 I. General I.1 Scientific Goals U
Page 10 and 11: 8 I. General massive protostars and
Page 12 and 13: 10 I. General achieve higher angula
Page 14 and 15: 12 I. General The institute is co-l
Page 16 and 17: 14 I. General Point Source sensitiv
Page 18 and 19: 16 I. General Edinburgh Groningen D
Page 20 and 21: 18 II. Highlights II.1 Light Echoes
Page 22 and 23: 20 II. Highlights 10 arcsec Fig. II
Page 24 and 25: 22 II. Highlights AB Dor C AB Dor A
Page 26 and 27: 24 II. Highlights II.3 The First He
Page 28 and 29: 26 II. Highlights II.4 Planetesimal
Page 30 and 31: 28 II. Highlights azimuthal directi
Page 32 and 33: 30 II. Highlights ticles fall faste
Page 34 and 35: 32 II. Highlights ANTU Seyfert 2 ga
Page 36 and 37: 34 II. Highlights cretion disk, one
Page 38 and 39: 36 II. Highlights 8.5 �m 2.3 ly 0
Page 40 and 41: 38 II. Highlights II.6 Massive Star
Page 42 and 43: 40 II. Highlights log M� /pc2 �
Page 44 and 45: 42 II. Highlights II.7 Observations
Page 46 and 47: 44 II. Highlights log IR luminosity
Page 48 and 49: 46 II.8 Dynamics, Dust, and Young S
Page 50 and 51: 48 II. Highlights and we see a temp
Page 52 and 53: 50 III. Selected Research Areas III
Page 54 and 55: 52 III. Scientific Work Fig. III.1.
Page 56 and 57: 54 III. Scientific Work the Data Ar
Page 60 and 61: 58 III. Scientific Work lower than
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Page 66 and 67: 64 III. Scientific Work z [AU] 1 0
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Page 72 and 73: 70 III. Scientific Work Column dens
Page 74 and 75: 72 III.3 The Galaxy-Dark Matter Con
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Page 78 and 79: 76 III. Scientific Work Mocking the
Page 80 and 81: 78 III. Scientific Work tion rate o
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Page 84 and 85: 82 III. Scientific Work DDO 53 HO I
Page 88 and 89: 86 III. Scientific Work tribution o
Page 90 and 91: 88 IV. Instrumental Development All
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106 IV. Instrumental Development IV
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108 IV. Instrumental Development jo
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110 V. People and Events 1� 10�
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112 V. People and Events V.2 Open H
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114 V. People and Events V.3 Furthe
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116 V. People and Events presented.
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118 V. People and Events The IMPRS
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120 V. People and Events Fig. V.5.2
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124 V. People and Events These oppo
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130 V. People and Events V.10 The P
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132 V. People and Events Abb. V.11.
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134 V. People and Events V.13 Four
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136 V. People and Events Lemke: I w
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138 V. People and Events Fig. V.13.
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140 V. People and Events V.14 Where
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142 V. People and Events against sp
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144 Staff Directors: Henning (Actin
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146 Working Groups Rainer Lenzen, H
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148 Cooperation with industrial Fir
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150 Conferences Conferences and Mee
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152 Conferences D. Lemke: JWST-miri
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154 Conferences/Service in Comitees
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156 Publications Szalay, I. Szapudi
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158 Publications Hamilton, C. M., W
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160 Publications Brinkmann, D. G. Y
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162 Publications Zheng, X. Z., F. H
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164 Publications of the wheel mecha
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166 Publications ASP Conf. Ser. 331
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The Max Planck Society The Max Plan
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